The present invention relates generally to safety starters, and more particularly to a fluorescent lamp starter protection circuit.
A fluorescent lamp starter circuit is essentially a time delay switch that allows a preheating circuit to warm the filaments at each end of a fluorescent lamp before the lamp is ignited. The most common automatic starter is a xe2x80x9cglow tube starterxe2x80x9d circuit that typically includes a glow-switch that is normally open. When current is applied to the glow tube starter circuit, the resulting glow discharge heats a bimetal contact which causes the contacts of the glow-switch to close a short time (1-2 seconds) thereby providing current to a preheating circuit and extinguishing the glow discharge. While the filaments are warming, the bimetal ultimately cools such that the contacts open thereby interrupting the current through the preheating circuit and producing an inductive xe2x80x9ckickxe2x80x9d through the ballast that should cause the lamp to ignite. However, the magnitude of the inductive kick is dependent upon the current supplied to the glow tube starter circuit and may at times be insufficient to ignite the lamp, thus requiring several successive attempts. Furthermore, the glow tube starter can cycle indefinitely if the lamp driven by the ballast is defective.
It is known to incorporate a pulse starter circuit in an attempt to improve the reliability of the glow tube starter circuit. A pulse starter circuit is designed to reduce the number of failed ignition attempts by electronically detecting the appropriate time to disengage the preheating circuit so as to optimize the inductive kick produced by the ballast. To increase the safety of the glow tube starter circuit, it is known to incorporate a thermal switch to disengage the glow tube starter circuit if an excessive number of ignition attempts is made, thereby eliminating the persistent blinking that occurs when a lamp cannot be started. Once disengaged, such existing xe2x80x9csafety starterxe2x80x9d circuits must be reset by means of a manual reset button in the luminaire, which is typically mounted on the ceiling. If a lamp has actually failed, this reset procedure can be accomplished while replacing the lamp. However, the occasional non-defective lamp is difficult to start simply because it is cold, so replacing the lamp is unnecessary. In such a situation, gaining access to the ceiling-mounted luminaire for the sole purpose of performing the manual reset procedure is extremely inconvenient and therefore disadvantageous.
Despite the improvements afforded by the pulse starter circuit and the safety starter circuit, existing glow tube starter configurations have additional disadvantages. The operation time (i.e., the elapsed time between the first and last attempt to ignite the lamp) is dependent upon the current level in the preheating circuit. Therefore, existing safety starters can only be implemented in lamp circuits that can maintain current levels that are high enough to produce temperatures that will trigger the thermal switch after the maximum allowable number of attempts to start the lamp has been made. Furthermore, the safety starter circuit is commonly configured such that the thermal switch is exposed to ambient environmental conditions (e.g., temperature and humidity), which at times results in sticking contacts that can cause dangerous failures due to overheating.
In spite of the recognized need, a continuing failure in the art has been an inability to provide an automatic starter that is reliable, safe, versatile, and easy to operate.
The circuit of the present invention fulfills the needs described above by implementing an automatic starter that includes a safety starter circuit, comprising a timer switch and a timer such as an electronic timer coupled to the timer switch and prevents the glow-switch from continually striking a lamp that has failed. The safety starter circuit can be configured to automatically reset whenever glow-switch cycling has ended either because the lamp has ignited, or because the safety starter circuit has ended the glow-switch cycling. Alternatively the timer can be reset when the supply voltage is switched on or off. The performance of the safety starter circuit is not affected by the ambient environment or the preheat current.
An exemplary embodiment of the present invention is a safety starter that controls the ignition of a fluorescent lamp. During operation a supply voltage provides power to a series arrangement comprising a ballast, lamp filaments and the safety starter. The safety starter comprises a series arrangement of a glow-switch and the timer switch. A control electrode of the timer switch is coupled to a timer such as a solid state timer. When both the glow-switch and the timer switch are conductive a preheat current flows through the filaments of the fluorescent lamp so that the lamp electrodes are preheated before a striking voltage is generated to ignite the lamp. The glow-switch allows current to flow through the lamp filaments long enough for the electrodes to be sufficiently heated. The contacts of the glow-switch subsequently open, thereby inducing a striking pulse intended to ignite the lamp. If the lamp is not ignited, the glow-switch cycles again. The timer and the timer switch limit the duration of the cycling of the glow-switch so that cycling does not occur beyond a predetermined maximum operation interval, such as the IEC maximum of five minutes. The solid state timer increments only while current flows through the glow-switch, a sensing resistor (that may be integral to the solid state timer) detecting the current flowing through the glow-switch. A capacitor that suppresses radio frequency (RF) interference is integrated in the safety starter. In this exemplary embodiment, the capacitor is connected in parallel with the glow switch.
In an alternative embodiment, the capacitor is connected in parallel with the fluorescent lamp. In this alternative embodiment the timer increments continuously, and the capacitor is not protected by the timer switch.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become more apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.